CN1791694A - Steel pipe for an airbag system and a method for its manufacture - Google Patents

Abstract

A high strength steel pipe for an airbag system has a steel composition comprising C: 0.05 - 0.20 %, Si: 0.1 - 1.0 %, P: at most 0.025 %, S: at most 0.010 %, Cr: 0.05 - 1.0 %, Al: at most 0.10 %, at least one of Ti and Mn satisfying (1) Ti 0.02% and (2) 0.4% Mn + 40Ti 1.2%, and a remainder of Fe. The composition may further include one or more of (i) at least one of Mo: 0.05 - 0.50 %, Ni: 0.05 - 1.5 %, V: 0.01 - 0.2 %, and B: 0.0003 - 0.005 %, (ii) at least one of Cu: 0.05 - 0.5 % and Nb: 0.003 - 0.1 %, and (iii) at least one of Ca: 0.0003 - 0.01 %, Mg: 0.0003 - 0.01 %, and REM: 0.0003 - 0.01 %. The steel pipe can be manufactured by forming a pipe from the above-described steel composition to obtain prescribed dimensions, heating to at least the Ac 1 transformation point and quenching, and then tempering at the Ac 1 transformation point or below.

Description

Steel pipe for airbag system and manufacturing method thereof

technical field

The present invention relates to a steel pipe having both high strength and high toughness with a tensile strength of over 1000 MPa and suitable for an airbag system. The invention also relates to a method of manufacturing such a steel pipe. In particular, the present invention relates to a thin-walled steel pipe for an airbag system having such high strength and high toughness that a brittle fracture surface does not appear in an internal pressure burst test at -40°C or even -80°C, and a manufacturing method thereof .

Background technique

In recent years, in the automobile industry, the introduction of devices that increase vehicle safety has been actively progressing. An example of such a device is an airbag system, which deploys an airbag with gas or the like between the passenger and the steering wheel or instrument panel before they collide with the passenger in the event of a collision, and absorbs the kinetic energy of the passenger to reduce injury.

Existing airbag systems use explosive chemicals to generate the gas used to deploy the airbag. However, other types of airbag systems that fill steel pipes with high-pressure gas for airbag deployment have been developed in consideration of the responsiveness of airbag deployment speed and air pressure adjustment, and are widely applicable.

In the latter type of airbag system, the gas for deployment is generally held at high pressure in a steel tube called an accumulator. In the event of a collision, the gas used for deployment in the accumulator is ejected into the airbag at once. As a result, a steel pipe used as a high-pressure gas accumulator for deployment is loaded with stress at a high strain rate in an extremely short time. Therefore, unlike simple structures such as conventional pressure cylinders and linear pipes, steel pipes used as pressure accumulators in airbag systems are required to have high dimensional accuracy, workability, and weldability, as well as high strength and Excellent blast resistance.

A steel pipe for an airbag system suitable for a pressure accumulator and a manufacturing method thereof are disclosed in, for example, JP-10-140238, JP-10-140249, JP-10-140250, JP-10-140283, JP- 10-212549, JP 2002-294339, JP 11-199929, JP 2001-49343, and JP 2002-194501.

The technologies described in the above-mentioned publications are all intended to manufacture high-strength and high-toughness steel pipes for airbag systems. However, the target tensile strength is 590 MPa or more, and the examples also show that the maximum tensile strength is only 947 MPa.

The technologies described in the above-mentioned publications can also obtain sufficient performance compared to conventional airbag systems. However, as automobiles have become increasingly lightweight in recent years, airbag systems have also been required to be smaller and lighter. Therefore, higher accumulator pressure and thinner steel pipes are also required today.

Contents of the invention

The present invention provides a steel pipe suitable for use as a high-strength and high-toughness airbag system (that is, for a pressure accumulator of the airbag system). This steel pipe satisfies various properties required in the above-mentioned current situation. The present invention also provides a method for manufacturing such a steel pipe.

The inventors of the present invention have found that in order to provide a steel pipe for an airbag system that has a high tensile strength and is excellent in blast resistance, and can sufficiently cope with increased accumulator pressure increases and thinner steel pipes, the steel pipe must have the following characteristics .

(a) Tensile strength of 1000 MPa or more, and

(b) In the pendulum impact test, a fracture surface exhibiting 100% ductility at least at -40°C, preferably at -60°C, more preferably at -80°C toughness of the fracture surface.

In addition, from the aspect of safety, it is also important to have good "blast resistance". This good blast resistance is achieved by the fact that no brittle fracture occurs in the internal pressure burst test by hydrostatic pressure at -40°C. , in addition, it was verified that the crack did not propagate throughout the entire length of the steel pipe. It is more desirable to show such burst resistance also in the internal pressure burst test at -80°C.

According to the present invention, a steel pipe having both a high strength of at least 1000 MPa and a high toughness of a ductile fracture surface at least at -40°C as verified in a pendulum impact test, and at the same time having the above-mentioned good blast resistance, Can be made practical by selecting a specific steel composition.

In the first aspect of the present invention, the steel pipe suitable for the airbag system (used to form the pressure accumulator of the airbag system) has a steel composition containing C: 0.05 to 0.20% and Si: 0.1 to 1.0% in mass % , P: 0.025% or less, S: 0.010% or less, Cr: 0.05 to 1.0%, Al: 0.10% or less, and one or both of Ti and Mn in amounts satisfying the following formulas (1) and (2), The remainder is composed of iron and impurities (wherein, the element symbol means the mass % of that element of the target).

Ti≤0.02% (1)

0.4≤Mn+40×Ti≤1.2 (2)

The steel pipe has a tensile strength of 1000 MPa or more.

The above-mentioned steel composition may further contain one or two or more of Mo: 0.05-0.50%, Ni: 0.05-1.5%, V: 0.01-0.2%, and B: 0.0003-0.005%.

This steel composition may further contain one or two of Cu: 0.05 to 0.5% and Nb: 0.003 to 0.1%.

This steel composition may further contain one or two or more of Ca: 0.0003 to 0.01%, Mg: 0.0003 to 0.01%, and REM: 0.0003 to 0.01%.

According to another aspect of the present invention, a method of manufacturing a steel pipe for an airbag system includes forming a steel pipe of a predetermined size from steel having the above-mentioned steel composition by a method including pipe making and subsequent cold working, and then forming the steel pipe that has been cold-worked After heating at a temperature above the Ac1 transformation point, it is rapidly cooled, and then tempered at a temperature below the Ac1 transformation point.

In a preferred embodiment of this method, the cold-worked steel pipe is heated at a temperature equal to or higher than the Ac3 transformation point, more preferably at a temperature in the range of 900 to 1000°C. This heating is preferably performed by rapid heating with a temperature increase rate of 10° C./sec or more, such as high-frequency induction heating. The subsequent rapid cooling is preferably performed at a cooling rate of 20°C/sec or more in a temperature range of at least 850°C to 500°C. Accordingly, it is possible to manufacture a steel pipe having a fine-grained structure with a gamma grain size of 11 or more after quenching (the larger the number, the finer the gamma grain size). Such a steel pipe exhibits extremely excellent blast resistance such that no significant crack growth is observed even in an internal pressure burst test at -80°C.

None of the patent publications listed above discloses a steel composition according to the present invention that satisfies the above (1) and (2), and satisfies high strength of 1000 MPa or higher and high toughness such as a ductile fracture surface at -40°C. The relationship between the tensile strength and the numerical value of (Mn+40×Ti) of typical steels disclosed in these patent publications is shown in FIG. 2 .

Description of drawings

FIG. 1 is a graph showing the relationship between the tensile strength in the circumferential direction of a steel pipe and vTrs100.

2 is a graph showing the relationship between the value of (Mn+40×Ti) and the tensile strength in the circumferential direction of the steel pipe disclosed in the examples of the present invention and the aforementioned patent publications.

Detailed ways

Next, the present invention will be described in more detail. In this specification, "%" showing a steel composition means "mass %" unless otherwise specified.

(A) Chemical composition of steel

According to the first aspect of the present invention, the steel pipe for an airbag system has the above-mentioned specific steel composition, and the steel composition has Ti and Mn contents satisfying the following formulas (1) and (2):

Ti≤0.02% (1)

0.4≤Mn+40×Ti≤1.2 (2)

The content of either Ti or Mn may be 0%.

The reason why the content ranges of the various elements in the specific steel composition of the present invention are limited as described above is as follows.

C: 0.05 to 0.20%

Carbon (C) is an inexpensive and effective element for increasing the strength of steel. If the content thereof is less than 0.05%, it will be difficult to obtain the required tensile strength of 1000 MPa or more. On the other hand, if it exceeds 0.20%, the workability and weldability will decrease. The preferable range of C content is 0.08-0.20%, and the more preferable range is 0.12-0.17%.

Si: 0.1 to 1.0%

Silicon (Si) is an element that improves the hardenability and strength of steel in addition to having a deoxidizing effect. In consideration of these effects of Si, the Si content is set to 0.1% or more. However, when the content exceeds 1.0%, toughness will fall. The preferable range of the Si content is 0.2 to 0.5%.

P: 0.025% or less

Phosphorus (P) can cause toughness reduction caused by grain boundary segregation. In particular, when the content thereof exceeds 0.025%, the toughness decreases significantly. The content of P is preferably 0.020% or less, more preferably 0.015% or less.

S: 0.010% or less

Sulfur (S) particularly reduces the toughness of the steel pipe in the T direction, that is, in the circumferential direction of the steel pipe. In particular, when the content thereof exceeds 0.010%, the toughness in the T-direction of the steel pipe significantly decreases. The S content is preferably 0.005% or less, more preferably 0.003% or less.

Cr: 0.05～1.0%

Chromium (Cr) is an element effective for improving the strength and toughness of steel. If its content is less than 0.05%, it is difficult to obtain a strength of 1000 MPa or more. However, if the content exceeds 1.0%, the toughness of the welded part will decrease. The preferred range of the Cr content is 0.2 to 0.8%, and the more preferred range is 0.4 to 0.7%.

Al: less than 0.10%

Aluminum (Al) is an element that has a deoxidizing effect and is effective in improving toughness and workability. However, if the Al content exceeds 0.10%, the generation of hairlines will be remarkable. The Al content is an impurity level, so the lower limit is not particularly limited, but it is preferably 0.005% or more. The preferable range of the Al content is 0.005 to 0.05%. The Al content in the present invention refers to the content of acid-soluble Al (so-called "sol.Al").

On the premise of this specific steel composition, in order to secure toughness as a steel pipe for an airbag system and obtain a strength of 1000 MPa or more, it is adjusted so that the Mn and Ti contents satisfy the above-mentioned formulas (1) and (2).

Ti: 0-0.02%

Titanium (Ti) may not be added to the steel composition of the present invention. When added, it is 0.02% or less so that the formula (1) is satisfied. The lower limit is not particularly specified, and the case where impurities are included to a certain extent is also included.

Ti is an element that has a deoxidizing effect if added. It also enhances the affinity with N, and exists stably as Ti nitride at high temperature. Therefore, grain growth during hot rolling is suppressed and contributes to toughness improvement. To obtain such an effect of Ti, the content of Ti is preferably 0.003% or more. However, if the Ti content exceeds 0.02%, the toughness will decrease instead. Therefore, when Ti is added, the content is preferably 0.003% to 0.02%.

Mn: 1.2% or less

Manganese (Mn) is an element that has a deoxidizing effect and is also effective in improving the hardenability of steel and improving the strength and toughness, so it can be contained at a maximum of 1.2%. If its content is less than 0.20%, sufficient strength and toughness may not be obtained. Therefore, the Mn content is preferably 0.20% or more. On the other hand, if the M content exceeds 1.0%, coarsening of MnS may occur, elongation during hot rolling, and a decrease in toughness may occur. Therefore, the Mn content is preferably 0.2% to 1.0%, more preferably 0.4% to 0.8%.

The contents of Ti and Mn are adjusted so as to satisfy the above formula (2). If the value of (Mn+40×Ti) is less than 0.4% or exceeds 1.2%, the desired high strength and/or high toughness cannot be obtained. The value of (Mn+40×Ti) is preferably not less than 0.6% and not more than 1.0%.

In the case of wanting to further improve the strength, blast resistance and/or weldability of the steel, in addition to the above-mentioned components, one of Mo, Ni, Cu, V, Nb, B, Ca, Mg and REM can also be added according to the situation. One or two or more are added to the steel composition of the steel pipe of the present invention in the range described later.

Mo, Ni, B, V

Molybdenum (Mo), nickel (Ni), boron (B), and vanadium (V) all have the effect of improving hardenability, so one or two of them may be added as optional components.

Mo also has the effect of increasing the strength by solid solution strengthening and precipitation strengthening. These effects of Mo can be obtained even at the impurity level content. However, in order to obtain the effect more remarkably, the Mo content is preferably 0.05% or more. However, if the content of Mo exceeds 0.50%, the welded part will be hardened and the toughness will decrease. Therefore, the content of Mo when added is preferably 0.05 to 0.50%, more preferably 0.1 to 0.35%.

Ni also has the effect of further improving the toughness. These effects of Ni can be obtained even at an impurity level content, but in order to obtain the effect more remarkably, the Ni content is preferably 0.05% or more. However, Ni is an expensive element, and especially when the content thereof exceeds 1.50%, the cost rises significantly. Therefore, the content of Ni at the time of addition is preferably 0.05 to 1.5%, more preferably 0.1 to 1.0%.

The hardenability-improving effect of B can be obtained even at the impurity level, but in order to obtain the effect more significantly, the B content is preferably 0.0003% or more. However, when the content of B exceeds 0.005%, toughness will fall. Therefore, the content of B at the time of addition is preferably 0.0003 to 0.005%. A more preferable range of the B content is 0.0003 to 0.002%.

V also has the effect of increasing the strength by strengthening through precipitation. Such an effect of V can be exhibited as long as it is contained in an amount of 0.01% or more, but if it exceeds 0.2%, the toughness will decrease. Therefore, the content of V at the time of addition is preferably 0.01 to 0.2%. A more preferable range of the V content is 0.03 to 0.10%.

Cu, Nb

Both copper (Cu) and niobium (Nb) have the effect of improving toughness, and therefore, one or both of them may be contained as optional components.

The toughness-improving effect of Cu can also be obtained at the impurity level. However, in order to obtain the effect more significantly, the Cu content is preferably 0.05% or more, more preferably 0.1%. However, Cu reduces the hot workability of steel, so when Cu is contained, Ni is also contained to ensure hot workability. In addition, when the content of Cu exceeds 0.5%, hot workability may not be ensured even if Ni is added in combination. Therefore, the content of Cu at the time of addition is preferably 0.05 to 0.5%.

The toughness-improving effect of Nb can also be obtained at the impurity level. However, in order to obtain the effect more significantly, the Nb content is preferably 0.003% or more, more preferably 0.005% or more. However, if the content of Nb exceeds 0.1%, the toughness will conversely decrease. The content of Nb at the time of addition is preferably 0.003 to 0.1%. A more preferable range of Nb content is 0.003 to 0.03%, and an even more preferable range is 0.005 to 0.02%.

Ca, Mg, REM

In order to further ensure better blast resistance of steel pipes for high-strength airbag systems, one or more of calcium (Ca), magnesium (Mg) and rare earth metal elements (REM) may be contained as optional components.

These elements all have the effect of improving the anisotropy of toughness, increasing the T-direction toughness of the steel pipe, and further improving the blast resistance. This effect can be obtained even at the impurity level content. However, in order to obtain the effect more remarkably, the content of any one element is preferably 0.0003% or more. However, if the content of any one element exceeds 0.01%, the inclusions will be clustered and hairline problems will occur. Therefore, the content when these elements are added is preferably 0.0003 to 0.01%, more preferably 0.0005 to 0.003%.

(B) Pipe making

In the present invention, in order to obtain a steel pipe for an airbag system, a seamless steel pipe or a welded pipe is manufactured using steel whose chemical composition has been adjusted as described above as a material. A seamless steel pipe is preferable from the viewpoint of reliability. The method of making seamless steel pipes, welded pipes, etc. is not particularly limited.

(C) cold working

The seamless steel pipe or welded pipe manufactured as described above is subjected to cold working under selected conditions so as to impart predetermined dimensional accuracy and surface texture to the steel pipe. The method of cold working does not matter as long as the specified dimensional accuracy and surface texture are obtained. Examples of useful cold working include cold drawing and cold rolling. The processing rate of cold working is not particularly specified, but the processing rate is preferably a reduction of area of 3% or more. In order to reduce the processing force of cold working, intermediate softening heat treatment can be applied before cold working.

(D) heat treatment

After the above-mentioned cold working in (C), the steel pipe is subjected to heat treatment for ensuring the required tensile strength while improving T-direction toughness and ensuring blast resistance. To make the steel pipe have high strength and blast resistance with a tensile strength of 1000 MPa or more, it is heated to a temperature above the Ac1 transformation point, quenched, and then tempered at a temperature below the Ac1 transformation point.

If the heating temperature before quenching is lower than the Ac1 transformation point, good T-direction toughness cannot be secured, and good blast resistance cannot be secured. The above-mentioned heating temperature is preferably a temperature above the Ac3 transformation point in the austenite region.

The scales formed on the surface of the steel pipe heated at high temperature for a long time increase, resulting in a decrease in dimensional accuracy and surface properties, and a decrease in blast resistance. Therefore, the above-mentioned heating is preferably performed after rapidly heating to a specified heating temperature and then maintained for a short time. This rapid heating is preferably performed at a temperature increase rate of 10°C/sec or higher. Such rapid heating can be realized by, for example, high-frequency induction heating and direct heating, but the heating method is not particularly limited. A preferred heating method is high frequency induction heating.

In particular, in the case of heating for such a short time, the heating temperature is preferably within the range of 900 to 1000°C, and most preferably within the range of 900 to 960°C. If the heating temperature is lower than 900°C, austenitization may not be complete during short-time heating, and a normal structure may not be obtained. When the heating temperature exceeds 1000° C., the γ grain size may become coarse and the toughness may decrease.

The heating gas when heated to a temperature above the Ac1 transformation point is desirably an environment in which the oxygen potential energy is as low as possible from the viewpoint of suppressing the occurrence of surface scales, and a reducing gas is more preferable.

Cooling after being heated to a temperature above the Ac1 transformation point, preferably above the Ac3 transformation point, adopts rapid cooling (specifically, in the temperature range of 850 to 500° C. at an average cooling rate of 5° C./second or more) so that The required tensile strength of 1000 MPa or more can be obtained stably and reliably. Preferably, the cooling rate is 20°C/sec or higher. Such quenching can be achieved by water quenching or the like.

After rapid heating to a temperature of 900-1000°C by high-frequency induction heating, rapid cooling is performed at a cooling rate of 20°C/s or more in a temperature range of 850-500°C, thereby preventing the coarsening of γ grains and stabilizing Realize a dense quenched structure characterized by a γ grain size (the grain size of prior austenite grains, measured by the Bechet-Beaujard method described in JIS G0551) after quenching as fine grains with a grain size of 11 or more. The steel pipe of the present invention having such a structure exhibits particularly good toughness and excellent blast resistance such that no significant crack growth is observed even in an internal pressure burst test at -80°C.

The steel pipe which has been quenched and cooled to near normal temperature is tempered at a temperature below the Ac1 transformation point in order to impart the required tensile strength of 1000 MPa or higher and blast resistance. There is no change in gamma particle size during this tempering. If the tempering temperature exceeds the Ac1 transformation point, it will be difficult to stably and reliably obtain the above properties. This tempering is preferably carried out by keeping in a temperature range of 450 to 700° C. for 10 minutes or more. After tempering, straighten the bend with a suitable straightener, etc.

In this way, according to the present invention, a steel pipe for an airbag system can be realized, which has a tensile strength of 1000 MPa or more, and has a temperature of -40°C or higher, preferably -60°C or higher, More preferably, it has high toughness such that it shows a 100% ductile fracture surface at -80°C or higher and does not show significant crack growth in an internal pressure burst test at -40°C, preferably -80°C. Therefore, according to the present invention, it is possible to provide a high-strength steel pipe for an airbag system that can sufficiently cope with an increase in the pressure of the accumulator and a reduction in the thickness of the steel pipe.

Example

Hereinafter, the effects of the present invention will be described in more detail based on examples. It should be considered that these examples are not limiting in any sense, but merely illustrative. The Ac1 transformation point of the steel used in the following examples is in the range of 700 to 760°C, and the Ac3 transformation point is in the range of 820 to 880°C.

Example 1

A steel billet having the chemical composition shown in Table 1 is used, heated to 1250°C, and then hot-processed into a tube with an outer diameter of 70 mm by piercing and rolling using a common Mannesmann piercer-continuous pipe rolling mill method. And the nominal size of the wall thickness is 4.1mm to manufacture seamless steel pipes. Next, the above-mentioned seamless steel pipe was cold-drawn to have an outer diameter of 60.33 mm and a wall thickness of 3.35 mm. Next, the steel pipe was heated at 920° C. for 10 minutes in a common walking furnace (heating rate: 0.3° C./second), quenched in water, and transformed in Ac1 in a common walking furnace (gas: air). Tempering is performed at a temperature below the point. In this way, three types of steel pipes for airbag systems were produced by varying the tensile strength for each steel composition by changing the tempering conditions. Water quenching is carried out so that the cooling rate in the temperature range of 850 to 500°C is 20°C or higher.

A certain length was cut out from each steel pipe, cut along the length of the pipe at room temperature, and unfolded. The pendulum test is carried out using a V-notch pendulum test piece with a width of 2.5 mm specified in JIS Z 2002 taken from the expanded pipe along its T direction. Using the No. 11 test piece specified in JIS Z 2201 taken along the T direction, the tensile test is carried out in accordance with the tensile test method for metal materials specified in JIS Z 2241. Table 2 and FIG. 1 show the relationship between the lower limit temperature (hereinafter referred to as vTrs100 ) and the tensile strength at which 100% of the ductile fracture surface obtained at this time can be ensured.

In addition, a burst test was carried out using steel pipes of 250 mm length cut out from the respective steel pipes. Both ends of this 250 mm long steel pipe were welded and closed, and the internal pressure was increased by a liquid at -40°C to burst the pipe. The blast resistance was evaluated by whether or not cracks propagated to either end of the blasted pipe. The results are also shown in Table 2.

Table 1 steel type Steel composition (mass%) Mn+40Ti C Si mn P S Cr Mo Cu Ni Nb Ti sol.Al A 0.11 0.30 1.62 0.015 0.002 0.82 - - - - 0.042 0.031 3.3* B 0.13 0.31 1.42 0.013 0.002 0.61 0.31 0.31 0.25 0.021 0.031 0.031 2.66* C 0.16 0.31 0.74 0.015 0.002 0.61 0.31 0.30 0.24 0.021 0.011 0.031 0.18 D. 0.15 0.30 0.50 0.014 0.002 0.61 0.31 0.31 0.25 0.021 0.007 0.032 0.78 E. 0.11 0.30 0.18 0.012 0.002 0.62 - - 0.07 0.008 0.005 0.031 0.38*

*Outside the scope of the invention

Table 2 steel type Steel pipe No. TS(MPa) vTrs100(°C) Brittle Crack Growth in Blasting Test at -40℃ A a 1085 -15 have b 1054 -20 have c 1005 -35 have B d 930 -40 none e 862 -55 none f 775 -60 none C g 1075 -40 none h 1035 -55 none i 1007 -65 none D. j 1102 -80 none k 1054 -100 none l 1008 -110 none E. m 800 -65 none no 762 -75 none o 684 -80 none

As shown in Table 2 and Fig. 1, the steel with high Mn content and (Mn+40Ti) value exceeding the range of formula (2) has vTrs100 higher than -40°C if the strength is made to be 1000 MPa or more. Therefore, steel pipes a, b, and c of steel A show brittle crack propagation in a hydrostatic pressure blast test at -40°C, and are not suitable as steel pipes for airbag systems. Steel pipes d, e, and f of steel B cannot have a strength of 1000 MPa or more if vTrs100≤-40°C is satisfied. On the other hand, steel E in which Mn is reduced and the value of (Mn+40Ti) is shifted to the lower limit side of the range of formula (2) cannot obtain a uniform quenched structure, and it is precisely because the strength of 1000 MPa or more cannot be obtained even if the quenching temperature is adjusted, so The strength levels of steel pipes d, e, and f of steel B cannot be achieved.

There is a tendency for vTrs100 to increase as the tensile strength increases. In steel type A, the tensile strength exceeds 1000 MPa, and vTrs100 greatly exceeds -40°C. However, among the steel pipes g to l of steel types C and D satisfying the above formulas (1) and (2) within the range of the specific steel composition, vTrs100 satisfies -40 in the region of tensile strength of 1000 MPa or more. ℃.

Example 2

Using a billet with the chemical composition shown in Table 3, after heating to 1250°C, through piercing and hot rolling using the usual Mannesmann piercer-continuous pipe rolling mill, the outer diameter of 70mm and the thickness of the wall are obtained. 4.1mm seamless steel pipe. This steel pipe was subjected to cold drawing processing (cold blanking processing) by a usual method to obtain an outer diameter of 60.33 mm and a wall thickness of 3.35 mm.

Steels 1 to 22 in Table 3 are steels whose composition satisfies the conditions specified in the present invention, and steels 23 to 27 are steels whose composition deviates from the conditions specified in the present invention.

The steel pipe produced by cold drawing was heated to 920° C. in a common walking furnace in the same manner as in Example 1. After maintaining the temperature for 10 minutes, water quenching was carried out, and thereafter, it was heated in a common walking furnace For tempering, heating was performed at a temperature not higher than the Ac1 transformation point for 30 minutes.

Each of the heat-treated steel pipes was subjected to a tensile test, a pendulum impact test, and a burst test.

The pendulum impact test was carried out in the same manner as in Example 1, using a V-notch pendulum test piece with a width of 2.5 mm specified in JIS Z 2202 obtained from the circumferential direction (T direction) of the steel pipe developed at room temperature, and the toughness was evaluated by vTrs100 .

The tensile test was carried out in accordance with the tensile test method for metal materials specified in JIS Z 2241 using the same material as the pendulum test piece, using the No. 11 test piece specified in JIS Z 2201.

In the internal pressure blasting test, five steel pipes with a length of 250 mm were cut out from the steel pipes, and both ends of the steel pipes with a length of 250 mm were welded and closed, and internal pressure was applied using a liquid to observe the crack growth at -40°C. In the test of five pipes, the explosion resistance was evaluated by the number of steel pipes in which cracks propagated to either end.

The results of the above tests are shown in Table 4.

Example 3

Seamless steel pipes having the compositions shown in Table 3 were produced in the same manner as in Example 2 except that the heat treatment conditions were changed.

In this example, the steel pipe produced by piercing, hot rolling and cold center processing as described in Example 2 was heated to 920°C at an acceleration of about 20°C/s by a high-frequency induction heating device. After the temperature of the tube reaches 920°C, it is kept for 5 seconds for high-frequency induction heating. Thereafter, water quenching was performed in the same manner as in Example 2, and then heating was performed for 30 minutes for tempering in a normal walk-in furnace.

For each steel pipe, the γ grain size of the steel was investigated by the JIS G0551 Bechet-Beaujard method. In addition, the tensile strength and vTrs were measured in the same manner as in Example 2. The internal pressure burst test was carried out in the same manner as in Example 1, except that it was carried out at a temperature of -80°C instead of -40°C, and was evaluated based on whether or not cracks in the blasted steel pipe extended to either end. These results are also shown in Table 4 at the same time.

As can be seen from Table 4, in Steel Nos. 1 to 22 having a steel composition in accordance with the present invention, when quenched by furnace heating as in Example 2, the tensile strength can also be made 1000 MPa or more, and the pendulum in the T direction The vTrs100 in the test was below -40°C, and the crack did not propagate to the end even in the burst test at -40°C. In addition, if quenching is carried out by rapid heating and short-time retention by high-frequency induction heating as in Example 3, the quenched structure is fine grains with a γ grain size of 11.0 or more, and generally speaking, the tensile strength is further improved. For example, vTrs is - The toughness is also further improved as shown below 90°C. As a result, no crack growth was observed in the -80°C burst test.

When the steel composition contains Mo, Ni, V, and B, the hardenability is better than when it is not contained, so it is easy to obtain a quenched and tempered structure with a uniform texture, and the balance between strength and toughness is excellent. Increase strength.

When the steel composition contains Cu, Ni, Ca, Mg, and REM, vTrs100 has a lower temperature and better toughness than when it does not contain it.

Steel No. 23 has a Mn content higher than the range of the present invention, does not satisfy the formula (2), and has lower toughness. Therefore, vTrs100 under furnace heating and quenching in Example 2 is -35°C, and the blast resistance is also lowered.

In Steel No. 24, the value of (Mn+40Ti) deviates from the formula (2) on the upper limit side, and the toughness is lowered. Therefore, the vTrs under furnace heating and quenching in Example 2 is -20°C, and the blast resistance is also lowered.

In steel No. 25, the value of (Mn+40Ti) deviates from the formula (2) on the lower limit side, and the tensile strength of 1000 MPa cannot be obtained even if the quenching temperature is adjusted.

In Steel No. 26, the Cr content was higher than the range of the present invention, and the weld toughness was lowered. Therefore, the vTrs under furnace heating and quenching in Example 2 was -20°C, and the blast resistance was lowered.

In Test No. 27, the Cr content was lower than the range of the present invention, and the hardenability was lowered. Therefore, a non-uniform structure was formed, and even if the quenching temperature was adjusted, 1000 MPa could not be obtained, and the blast resistance was not satisfactory.

Even if the steel of the above comparative example was quenched by high-frequency induction heating as in Example 3, vTrs could not reach -80°C or lower, and crack growth was observed in the blast test at -80°C.

Fig. 2 is for comparing the present invention with the prior art, and represents the (Mn+40Ti ) The relationship between the value and the tensile strength. As shown in Figure 2, due to the satisfaction of formula (2), the strength can be increased to a level exceeding 1000Mpa.

As mentioned above, although this invention was demonstrated about a suitable form, these are only illustrations and do not limit this invention. Various changes can be made to the modes described above without departing from the scope of the present invention, which should be conceivable by those skilled in the art.

table 3 Steel No. Steel composition (mass%) Mn+40Ti C Si mn P S Cr Ti al Mo Ni V B Cu Nb Ca Mg REM 1 0.15 0.31 0.81 0.008 0.001 0.60 0.008 0.035 - - - - - - - - - 1.13 2 0.15 0.30 0.55 0.015 0.002 0.56 0.001 0.035 0.29 - - - - - - - - 0.59 3 0.14 0.31 0.45 0.015 0.002 0.52 0.002 0.029 - 0.22 - - - - - - - 0.53 4 0.16 0.29 0.53 0.011 0.003 0.64 0.009 0.032 - - 0.05 - - - - - - 0.89 5 0.16 0.34 0.41 0.012 0.003 0.61 0.011 0.033 - - - 0.0011 - - - - - 0.85 6 0.09 0.31 0.47 0.009 0.002 0.60 0.013 0.033 0.32 0.25 - - - - - - - 0.99 7 0.12 0.35 0.44 0.011 0.004 0.12 0.012 0.028 0.31 0.24 0.03 - - - - - - 0.92 8 0.15 0.32 0.45 0.008 0.003 0.56 0.011 0.033 0.17 - - 0.0021 - - - - - 0.89 9 0.16 0.35 0.45 0.009 0.003 0.58 0.013 0.025 - - - - 0.33 - - - - 0.97 10 0.13 0.27 0.43 0.012 0.003 0.55 0.011 0.022 - - - - - 0.018 - - - 0.87 11 0.14 0.33 0.51 0.012 0.003 0.21 0.009 0.035 0.33 - - - 0.31 - - - - 0.87 12 0.15 0.30 0.50 0.008 0.001 0.60 0.011 0.035 0.32 0.24 - - 0.32 0.025 - - - 0.94 13 0.15 0.30 0.51 0.015 0.002 0.62 0.008 0.029 - - - - - - 0.0023 - - 0.83 14 0.14 0.32 0.44 0.013 0.002 0.66 0.009 0.032 - - - - - - - 0.0008 - 0.80 15 0.16 0.29 0.55 0.012 0.003 0.64 0.011 0.033 - - - - - - - - 0.0015 0.99 16 0.14 0.33 0.49 0.011 0.003 0.33 0.011 0.022 - - - - - - 0.0015 0.0012 - 0.93 17 0.16 0.34 0.52 0.012 0.003 0.59 0.013 0.033 0.31 - - - - - 0.0021 - - 1.04 18 0.09 0.36 0.47 0.009 0.002 0.60 0.012 0.028 0.15 - 0.05 0.0022 - 0.018 - - - 0.95 19 0.16 0.36 0.51 0.011 0.004 0.60 0.011 0.033 0.29 - - - 0.23 0.010 0.0022 - - 0.95 20 0.15 0.32 0.45 0.008 0.003 0.56 0.013 0.025 - - - - 0.25 0.021 0.0021 0.0011 - 0.97 twenty one 0.16 0.33 0.43 0.009 0.003 0.58 0.011 0.022 0.30 0.25 - - 0.30 0.022 0.0023 - - 0.87 twenty two 0.14 0.34 0.45 0.015 0.003 0.63 0.013 0.025 0.30 0.31 0.04 - 0.18 0.008 0.0012 0.0008 0.0007 0.97 twenty three 0.15 0.31 1.51 0.008 0.001 0.60 0.009 0.035 - - - - 0.31 0.019 - - - 1.87* twenty four 0.16 0.30 0.62 0.015 0.002 0.56 0.019 0.035 0.29 - - - - - - - - 1.38* 25 0.12 0.31 0.23 0.008 0.001 0.56 0.004 0.035 - - - - - - - - - 0.39* 26 0.15 0.30 0.70 0.015 0.002 1.54* 0.011 0.035 - - 0.02 - - - - - - 1.14 27 0.15 0.30 0.65 0.005 0.002 -* 0.011 0.035 0.29 - - - - 0.021 - - - 1.09

*Outside the scope of the invention

Table 4 Steel No. Embodiment 2: Furnace heating and quenching (920°C×10 minutes) Embodiment 3: High-frequency induction heating and quenching (920 ° C × 5 seconds) Remark TS(MPa) vTrs100(°C) Blasting performance at -40°C1 ⁾ gamma granularity TS(MPa) vTrs100(°C) Blasting performance at -80°C2 ⁾ 1 1011 -40 0 11.0 1023 -90 none Invention example 2 1011 -40 0 12.0 1050 -100 none 3 1005 -40 0 11.5 1021 -100 none 4 1012 -40 0 12.0 1025 -100 none 5 1008 -45 0 12.0 1026 -100 none 6 1025 -65 0 11.5 1035 -110 none 7 1033 -65 0 12.0 1045 -110 none 8 1015 -45 0 12.0 1021 -100 none 9 1022 -50 0 11.5 1037 -90 none 10 1015 -70 0 12.0 1023 -100 none 11 1053 -70 0 11.5 1017 -110 none 12 1073 -80 0 12.5 1112 -120 none 13 1015 -50 0 11.0 1010 -100 none 14 1013 -45 0 11.0 1012 -90 none 15 1011 -45 0 11.0 1019 -90 none 16 1021 -50 0 11.5 1030 -90 none 17 1053 -50 0 11.5 1070 -90 none 18 1056 -70 0 11.5 1086 -90 none 19 1071 -100 0 12.0 1120 -120 none 20 1087 -80 0 12.5 1134 -110 none twenty one 1131 -80 0 12.5 1162 -90 none twenty two 1150 -80 0 12.5 1170 -90 none twenty three 1023 -15 4 11.5 1058 -60 have comparative example twenty four 1008 -20 3 11.0 1027 -60 have 25 840 -40 0 10.5 984 -60 have 26 1005 -20 3 12.5 1180 -60 have 27 765 -35 1 10.0 954 -70 have

¹⁾ Among the 5 steel pipes tested, the number of steel pipes whose cracks extended to any end,

²⁾ Whether the crack of the tested steel pipe has extended to any end

Claims (12)

1. A steel pipe for an airbag system, having a tensile strength of 1,000 MPa or more, the steel composition of which is: 0.05 to 0.20% of C, 0.1 to 1.0% of Si, 0.025% or less of P, 0.010 % or less of S, 0.05 to 1.0% of Cr, 0.10% or less of Al, at least one of Ti and Mn satisfying the following formulas (1) and (2), 0 to 0.50% of Mo, 0 to 1.5% of Ni, 0-0.2% V, 0-0.005% B, 0-0.5% Cu, 0-0.1% Nb, 0-0.01% Ca, 0-0.01% Mg, 0-0.01% REM , the rest is basically composed of iron and impurities, Ti≤0.02% (1) 0.4≤Mn+40×Ti≤1.2 (2) However, the symbol of the element in Formula (1) represents the mass % of the said element. 2. The steel pipe for an airbag system according to claim 1, wherein the steel composition contains 0.2% by mass or more of Mn. 3. The steel pipe for an airbag system according to claim 1, wherein said steel composition contains 0.05 to 0.50% of Mo, 0.05 to 1.5% of Ni, 0.01 to 0.2% of V, and 0.0003 to 0.005% of B in terms of mass % 1 or more of them. 4. The steel pipe for an airbag system according to claim 1, wherein the steel composition contains one or both of 0.05 to 0.5% of Cu and 0.003 to 0.1% of Nb by mass %. 5. The steel pipe for an airbag system according to claim 1, wherein the steel composition contains one or two of 0.0003 to 0.01% of Ca, 0.0003 to 0.01% of Mg, and 0.0003 to 0.01% of REM in mass % above. 6. The steel pipe for an airbag system according to any one of claims 1 to 5, which has a steel structure with a γ grain size of 11 or more. 7. A method of manufacturing a steel pipe for an airbag system, characterized in that: the steel composed of the steel according to any one of claims 1 to 5 is formed into a steel pipe by a method including pipe making and subsequent cold working, and then, The cold-worked steel pipe is heated at a temperature equal to or higher than the Ac1 transformation point, then rapidly cooled, and then tempered at a temperature equal to or lower than the Ac1 transformation point. 8. The method according to claim 7, wherein the heating temperature of the cold-worked steel pipe in the heating step is a temperature equal to or higher than the Ac3 transformation point. 9. The method according to claim 8, wherein the heating temperature is in the range of 900-1000°C. 10. The method according to claim 7, wherein the heating is performed by rapid heating with a temperature increase rate of 10° C./second or more. 11. A method according to claim 10, heating by high frequency induction heating. 12. The method according to any one of claims 7-11, wherein the rapid cooling is performed at least in the temperature range of 850-500°C at a cooling rate of 20°C/sec or more.